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Shifting the Paradigm: Redefining the Chronostratigraphy of the Triassic Rewan Group, Bowen Basin, Australia
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DOI：10.1111/bre.70102.png)
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Abstract
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The Triassic Rewan Group in the northern Bowen Basin, Queensland, Australia preserves a key record of terrestrial environments and faunas that have been assumed to document recovery following the end-Permian mass extinction (EPME). The Rewan Group, consisting of the Sagittarius Sandstone and the Arcadia Formation, accumulated in a retroarc foreland basin during the Hunter–Bowen Orogeny, but its chronostratigraphy has remained poorly constrained because previous age models relied mainly on limited, geographically restricted biostratigraphy (esp. palynostratigraphy). Here, we couple detailed lithostratigraphic analysis with high-density U–Pb detrital-zircon (DZ) geochronology by LA-ICP-MS (ca. 300 analyses per sandstone), calibrated against latest Permian (ca. 252–251 Ma) tuff and tuffaceous reference horizons, and a detrital apatite U–Pb dataset, to establish a robust chronostratigraphic framework for the Sagittarius Sandstone and Arcadia Formation. We compare several maximum depositional age (MDA) metrics and show that maximum-likelihood ages (MLA) at 10% discordance provide a stratigraphically coherent MDA estimate, with younger single-grain and cluster-based estimators used as internal checks and minimum bounds. Preferential zircon picking further shows that targeted grain selection enriches the youngest Triassic populations, strengthening the robustness of the resulting MDA constraints. The resulting MDAs demonstrate that the Rewan Group spans ca. 250–233 Ma (Olenekian to Carnian) and that the lower Rewan Group contact is strongly time-transgressive. In the Taroom Trough (foredeep), fluvial successions show an overall up-section younging from the latest Permian reference ages through Olenekian–Anisian Sagittarius Sandstone into the late Ladinian–earliest Carnian Arcadia Formation (based on sandstone MDAs). In contrast, in the Denison Trough (back-bulge), the latest Permian coal measures are directly overlain by Middle–early Late Triassic Rewan Group deposits, implying a hiatus or condensed interval of at least ca. 12–15 Myr based on MDAs. We show that known Arcadia Formation vertebrate fossil-bearing horizons are late Ladinian (239 Ma) to early Carnian (236 Ma) rather than earliest Triassic, with the younger date also corroborated by a detrital apatite lower-intercept age of ca. 239 Ma. These revised ages show that the Arcadia Formation vertebrate assemblages do not come from the immediate post-EPME interval but from the late Ladinian to early Carnian, across the onset of the Carnian Pluvial Episode.
Keywords:
Bowen Basin
chronostratigraphy
detrital-zircon
geochronology
retroarc foreland basin
Rewan group
Triassic
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